In coastal foredunes, Ammophila arenaria (L.) Link grows vigorously when it is buried regularly by windblown sand and degenerates at stabilized sites. Nematodes and soil-borne fungi were found to be involved in its decline. In order to establish their role in the disease complex, seedlings of A. arenaria were inoculated with several groups of potentially harmful soil organisms that were isolated from its root zone. Inoculation of single species of fungi did not reduce the growth of the seedlings, but combining all fungi that were commonly found in the Dutch coastal foredunes significantly reduced growth to about 80% of that in sterilized soil. This indicates synergistic effects between commonly found plant pathogenic fungi. The addition of large numbers of the nematode Telotylenchus ventralis, the only species that could be successfully grown on A. arenaria in the laboratory, reduced plant growth to the same level as in non-sterile soil, but numbers were 80 times greater than in the latter soil. Inoculation with relatively large numbers of T. ventralis in combination with the commonly occurring fungi reduced plant growth in sterilized soil to a level similar to that in non-sterile soil. The involvement of other species of nematodes, such as Heterodera spp. or Meloidogyne maritima, in the decline of A. arenaria in non-sterile soil could not be proven in inoculation experiments with sterile soil, but it is likely that these species may also be involved. It is concluded, therefore, that several different combinations of soil organisms can be harmful to A. arenaria, so that natural decline is not caused by one simple well defined pathosystem.